A TMR sensor otherwise known as a magnetic tunneling junction (MTJ) is a key component in magnetic devices such as Magnetic Random Access Memory (MRAM) and a magnetic recording head. A TMR sensor typically has a stack of layers with a configuration in which two ferromagnetic layers are separated by a thin non-magnetic insulator layer. The sensor stack in a so-called bottom spin valve configuration is generally comprised of a seed (buffer) layer, anti-ferromagnetic (AFM) layer, pinned layer, tunnel barrier layer, free layer, and capping layer that are sequentially formed on a substrate. The free layer serves as a sensing layer that responds to external fields (media field) while the pinned layer is relatively fixed and functions as a reference layer. The electrical resistance through the tunnel barrier layer (insulator layer) varies with the relative orientation of the free layer moment compared with the reference layer moment and thereby converts magnetic signals into electrical signals. In a magnetic read head, the TMR sensor is formed between a bottom shield and a top shield. When a sense current is passed from the top shield to the bottom shield (or top conductor to bottom conductor in a MRAM device) in a direction perpendicular to the planes of the TMR layers (CPP designation), a lower resistance is detected when the magnetization directions of the free and reference layers are in a parallel state (“1” memory state) and a higher resistance is noted when they are in an anti-parallel state or “0” memory state. Alternatively, a TMR sensor may be configured as a current in plane (CIP) structure which indicates the direction of the sense current.
A giant magnetoresistive (GMR) head is another type of memory device. In this design, the insulator layer between the pinned layer and free layer in the TMR stack is replaced by a non-magnetic conductive layer such as copper.
In the TMR stack, the pinned layer may have a synthetic anti-ferromagnetic (SyAF) configuration in which an outer pinned layer is magnetically coupled through a coupling layer to an inner pinned layer that contacts the tunnel barrier. The outer pinned layer has a magnetic moment that is fixed in a certain direction by exchange coupling with the adjacent AFM layer which is magnetized in the same direction. The tunnel barrier layer is so thin that a current through it can be established by quantum mechanical tunneling of conduction electrons.
A TMR sensor is currently the most promising candidate for replacing a GMR sensor in upcoming generations of magnetic recording heads. An advanced TMR sensor may have a cross-sectional area of about 0.1 microns×0.1 microns at the air bearing surface (ABS) plane of the read head. The advantage of a TMR sensor is that a substantially higher MR ratio can be realized than for a GMR sensor. In addition to a high MR ratio, a high performance TMR sensor requires a low areal resistance RA (area×resistance) value, a free layer with low magnetostriction (λ) and low coercivity (Hc), a strong pinned layer, and low interlayer coupling (Hin) through the barrier layer. The MR ratio (also referred to as TMR ratio) is dR/R where R is the minimum resistance of the TMR sensor and dR is the change in resistance observed by changing the magnetic state of the free layer. A higher dR/R improves the readout speed. For high recording density or high frequency applications, RA must be reduced to about 1 to 3 ohm-um2.
A MgO based MTJ is a very promising candidate for high frequency recording applications because its tunneling magnetoresistive (TMR) ratio is significantly higher than for AlOx or TiOx based MTJs as demonstrated by S. Yuasa et al. in “Giant room-temperature magnetoresistance in single crystal Fe/MgO/Fe magnetic tunnel junctions”, Nature Materials, 3, 868-871 (2004), and in “Giant tunneling magnetoresistance up to 410% at room temperature in fully epitaxial Co/MgO/Co magnetic tunnel junctions with bcc Co(001) electrodes”, Appl. Phys. Lett., 89, 042505 (2006), and by S. Parkin et al. in “Giant tunneling magnetoresistance at room temperature with MgO (100) tunnel barriers”, Nature Materials, 3, 862-867 (2004).
CoFeB has been used in the free layer for MgO based MTJs to achieve high MR ratio and a soft magnetic layer. D. Djayaprawira et al. showed that MTJs with a CoFeB/MgO(001)/CoFeB structure made by conventional sputtering can also have a very high MR ratio of 230% with advantages of better flexibility and uniformity in “230% room temperature magnetoresistance in CoFeB/MgO/CoFeB magnetic tunnel junctions”, Physics Letters 86, 092502 (2005).
For a low RA application, the MR ratio of CoFeB/Mg/MgO/CoFeB MTJs can reach 138% at RA=2.4 ohm/μm2 according to K. Tsunekawa et al. in “Giant tunneling magnetoresistance effect in low resistance CoFeB/MgO(001)/CoFeB magnetic tunnel junctions for read head applications”, Applied Physics Letters 87, 072503 (2005). In this case, a DC-sputtered Mg layer was inserted between the CoFeB pinned layer and an RF-sputtered MgO layer, an idea initially proposed by T. Linn et al. in U.S. Pat. No. 6,841,395 to prevent oxidation of the bottom electrode (CoFe) in a CoFe/MgO (reactive sputtering)/NiFe structure. Also, a Ta getter pre-sputtering prior to RF sputtering a MgO layer can achieve 55% TMR with 0.4 ohm/μm2 as reported by Y. Nagamine et al. in “Ultralow resistance-area product of 0.4 ohm/μm2 and high magnetoresistance above 50% in CoFeB/MgO/CoFeB magnetic junctions”, Appl. Phys. Lett., 89, 162507 (2006).
In order to achieve a smaller Hc but still maintain a high TMR ratio, the industry tends to use CoFeB as the free layer in a TMR sensor. Unfortunately, the magnetostriction (λ) of a CoFeB free layer is considerably greater than the maximum acceptable value of about 5×10−6 for high density memory applications. A free layer made of a CoFe/NiFe composite has been employed instead of CoFeB because of its low λ and soft magnetic properties. However, when using a CoFe/NiFe free layer, the TMR ratio will degrade. Another approach is a composite free layer containing CoFeB with a positive λ and a NiFe layer with a negative λ to result in a low λ and magnetic softness for the free layer. However, a CoFeB/NiFe type free layer structure is not usable because direct contact of CoFeB with NiFe will cause a drastic drop in the MR (TMR) ratio. Thus, an improved free layer in a TMR sensor is needed that provides low magnetostriction in combination with a high TMR ratio, low RA value, and low coercivity.
U.S. Pat. No. 7,333,306 and U.S. Patent Application 2007/0047159 show a tri-layered free layer represented by CoFe/CoFeB/NiFe to achieve low coercivity and low magnetostriction for either GMR-CPP or TMR sensors.
In U.S. Patent Application No. 2007/0139827, a free layer is described that includes a sense enhancing layer (Ta) sandwiched between a first ferromagnetic layer and a second ferromagnetic layer. The first ferromagnetic layer has a positive magnetostriction and is made of CoFeB or CoFe based alloys while the second ferromagnetic layer has a negative magnetostriction and is comprised of CoFe, Ni, or NiFe based alloys.
U.S. Patent Application No. 2007/0188942 discloses a free layer comprised three layers that include a lower NiFe or CoFe layer on the tunnel barrier layer, a Ta, Ru, Cu, or W spacer, and a CoFeB, CoFe, or NiFe upper layer.
In U.S. Patent Application 2007/0242396, a free layer is disclosed that comprises FeCo, a Heusler alloy, and NiFe.
U.S. Patent Application No. 2008/0061388 discloses a free layer with a CoFeB/Ru/CoFeTaB configuration.
U.S. Pat. No. 6,982,932 and U.S. Patent Application 2008/0152834 describe free layers that are laminations of NiFe, CoFe, and CoFeB.